Michelson and Sagnac interferometers are well known. A Michelson interferometer, such as shown in FIG. 1, comprises a partially reflecting mirror 10 and a pair of reflecting mirrors 21, 22. The partially reflecting mirror 10 divides the light into two components. The mirrors 21, 22 and partially reflecting mirror 10 are arranged so that light passes along two linear paths, namely legs 31, 32. Each component travels a return path along a different one of the legs. The optical path length along one of the legs 32 is adjustable. The components recombine at a focal plane to form fringes. The components are combined through a lens 40, arriving at the lens at equal angles of inclination. By displacing one of the reflecting mirrors 22 in the direction of the beam, the optical path difference between the two legs can be varied. The fringes 55 usually take the form of concentric rings. As the path difference is changed the separation between fringes becomes greater or smaller. If the path length difference is varied linearly with time, and a detector 50 is placed at the centre of the annuli of fringes 55, the signal from the detector will vary sinusoidally with a period determined by the wavelength and path difference.
One of the advantages of the Michelson interferometer is that it can accept input rays incident over a relatively wide angular range. However, in the interference fringes any spatial information about the source is lost because it is distributed uniformly in the interference rings.
A Sagnac interferometer is shown in FIG. 2. The Sagnac interferometer also comprises a partially reflecting mirror 110, and a pair of reflecting mirrors 121, 122. The partially reflecting mirror divides the light, as for the Michelson interferometer, but instead of the two components travelling along legs the position of the reflecting mirrors is changed such that the two components are not reflected directly back to the partially reflecting mirror 110 but are reflected to the other reflecting mirror 121 or 122. Hence, the components travel along similar enclosed paths but in opposite directions. For this reason the Sagnac interferometer is sometimes known as a common path interferometer. The two components exit via the partially reflecting mirror 110 and through lens 140 to produce fringes 155. Different to the Michelson, the fringes 155 are linear rather than circular.
If a detector 150 is placed in the interference pattern, and one of the mirrors 122 is scanned as shown by the arrow in FIG. 2, the signal from the detector will vary sinusoidally. Alternatively, moving a detector across the interference pattern, the signal will also vary sinusoidally.
Another difference between the Sagnac and Michelson interferometers is that the Michelson requires very accurate positioning of the mirrors for each arm, whereas the position of the mirrors for the Sagnac device is more tolerant because the path difference is produced as a result of the triangular path of the beams being asymmetric and providing shear of the two beams.
Interferometers based on the Sagnac arrangement described above have been reported by Berlinghieri J. C. et al., “A CCD Fourier Transform Spectrometer”, CCDs in Astronomy: Proceedings of the Conference, Tucson, Ariz., 6-8 Sep., 1989. The device reported used a CCD array to detect the interference pattern. Okamoto T., “Fourier Transform Spectrometer with Self-Scanning Photodiode Array”, Applied Optics, Vol. 23, No. 2, 1984, describes a similar device but incorporates an additional mirror to fold the output beam towards the detector. Okamoto also considers that the optical throughput of this system is larger than that of Michelson type interferometers, because in such interferometers, the resolving power is limited by the extent of the source. Lucey P. G. et al. “SMIFTS: A Cryogenically Cooled, Spatially Modulated Imaging Infrared Interferometer Spectrometer”, Proc. SPIE, Vol. 1937, 130 (1993) also describes a similar device, but additionally includes a cylindrical lens to reimage the input aperture at the detector.
Other interferometer configurations have also been considered such as the DASI (Digital Array Scanned Interferometer) discussed by Katzberg and Statham in NASA Technical paper 3570, August 1996. This device comprises a Wollaston prism and a detector array.
Many of the above devices are cumbersome, require tight control of position and quality of components, and cannot provide a real-time output spectrum.